Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (“MRI”). Particular embodiments relate to acquisition of anisotropic three-dimensional magnetic resonance images with ultrashort echo times (“3DUTE MRI”).
Discussion of Art
When a substance such as human tissue is subjected to a uniform magnetic field (a polarizing field B0), the individual magnetic moments of particle spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to an RF magnetic field (excitation field B1), which defines an x-y plane and varies at a frequency near a Larmor frequency corresponding to spins of selected particles, the net aligned moment, or “longitudinal magnetization” (M2) of those selected particles, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M. After the excitation signal B1 is terminated, the tipped spins “relax” back into the precession defined by the polarizing field, and by doing so, produce RF signals. The time from termination of the excitation signal B1, until initiation of another excitation signal B1, is known as an “echo time,” during which the relaxation RF signals may be received and processed as k-space data from which an image may be formed. In order to form a pixelated image that a human doctor can interpret, gradient magnetic fields (Gx, Gy, Gz) are applied to localize the tissue response to the excitation signal B1. The gradient fields may be applied during, or after, the excitation signal B1.
For certain tissue types, e.g., musculoskeletal tissues, it is desirable to obtain an image with ultrashort echo times (“UTE”), on the order of less than 100 μs.